iPhone 6 WiFi

My IPhone 6 software updated iOS 12 doesn’t have WiFi calling setting then where is the question of enabling it? Is there any solution as I need it for voice call facility through jio fibre. Many people are answering how to enable it. The point is how is it some phones have WiFi setting provision and I don’t have it in settings either under phone or cellular setting. Is there any solution to create the setting.